Self Assessment error or mistake


First of all, a Self-Assessment error or mistake can result in you missing the deadline. If this occurs, HM Revenue & Customs (HMRC) may impose a fine on. If your return is wrong the first time around, you will also need to spend some time doing this again.

Most owners of small businesses be they a sole trader, in a partnership or a director of a limited company, will need to complete a Self-Assessment (SA) Income Tax Return each year.

If you are new to Self-Assessment (SA), you may wonder what is a Self-Assessment tax return. If you are required to file a tax return, you have the choice of doing this yourself or you might consider looking for a Self-Assessment tax return accountant near me, to look after your tax affairs.

When you do complete the SA yourself, you will need to know how to fill in a Self-Assessment tax return.

Initial thoughts

As a UK taxpayer, if you are both:

  • you will have a personal tax bill in the current tax year

-you need to file your personal Self-Assessment (SA) Tax Return every twelve months. Save

When filing your tax return, besides reporting hour taxable income and paying the tax that you owe, you may also be required to make payments on account. If you have a good contractor accountant, they will help guide you through this process.

HMRC online personal tax account

It is a good idea to register online for a personal tax account with HMRC. When you have this in place, you can view your tax liabilities and payments as well as any due dates for each tax year. You will also have access to view our PAYE coding notices, therefore you can see your latest tax code and if there is something in here that should not be, you or your accountant con contact HMRC and ask them to correct this.

What to consider   

If you need to file a UK tax return, this will be an annual task. The tax year covers 6 April to the next 5 April. What’s more, you need to file this with HMRC by the following 31 January after the 5 April. Therefore, for the tax year ended 5 April 2022, the due date will be 31 January 2023.

Sad as it is, many people always leave this until the last minute. As a result, this is where an error or mistake can quite often occur, if you do not prepare yourself. If your return is not correct, you may also pay extra tax than what is actually due.

Best practice is to file your personal tax return early each year. We would advise that you check through this from start to finish, once it is complete.

You will then have peace of mind, with the knowledge that you are fulfilling your duties correctly and on time.

What’s more, a Self-Assessment error can often arise if you do not check your tax return. There are some common reasons for such errors, and we will show these below.

Unique Taxpayer Reference UTR 

You will need to make sure you show your correct UTR number on your tax return. In addition, you will also need to ensure that your National Insurance number is correct. If either of these are incorrect on your tax return it will be void this when you submit it.

As an individual, you will receive your NI number before you begin work in the UK. When you have been working, your NI number will also show on any previous payslips and tax documents that you receive from HMRC.

Your UTR will show on last year’s tax return and also on any SA correspondence from HMRC.

SA error or mistake -imprecise records 

It is important, that you keep records of your income and any reliefs safe throughout the course of the tax year.

To help avoid an error or mistake, you should keep in a safe place the details of your:

  • Salary -this is shown on form P60 or form P45 as well as your payslips.
  • Dividends -from your own company and any others. Any dividends from other companies should show on dividend vouchers that they will send to you.
  • Benefits in kind -these will show on form P11D. Benefits will include company car benefits, medical insurance, beneficial loan, and others.
  • Bank interest on personal bank accounts. This does not include ISAs.
  • Pension income -state pension or personal pension(s).
  • Government benefits such as universal credit, employment and support allowance (ESA) and income support.
  • Rental property income details -this includes the total of both your rental income and any expenses in relation to this.
  • Donations under Gift Aid.
  • Any personal pension contributions.
  • The details with regards to child benefit received -you will receive thirteen four weekly payments each year, if you receive this.
  • Payments into an employee share scheme.
  • Investments under Venture Capital Trust schemes, such as EIS.

Various examples of SA errors or mistakes

If you miss the filing deadline 

A very common mistake is missing the deadline and this is quite common. It is also more likely to occur if you leave this until close to the filing deadline. Many accountants and tax experts are put under pressure by their clients who send them their tax return information at the very last minute. It may then become clear, when you prepare your return, that you have not included some information on this or something else is incomplete. In turn, this could lead to you not being able to file your return on time.

If you file the return yourself via the HMRC website, please do not forget to save your return, as you go along. Please also make sure that you also save it after you complete all of the questions. What’s more, please also make sure that the return is sent online via their website. Once this has been done, they will confirm the receipt of this to you. 

Report your income incorrectly 

Whoever will prepare your tax return, you or your accountant, please make sure that you double-check that the figures are correct. This can be another Self-Assessment error if you do not check everything and something on your return is not right. You will need to make sure that you include all of the figures for your income and reliefs. Income is typically salary and dividends and any other income. Reliefs are payments under Gift Aid, personal pension payments and payments under Venture Capital Trust schemes such as EIS. If your accountant prepares your return on your behalf, please make sure that you check all of the details are correct before you sign this off.

Failure to declare all of your income 

You need to make sure that you include all of your taxable income on your tax return. Your income will include both income or Capital Gains. Certain types of income that you may receive are not taxable. These items include ISAs, UK Government Gilts, Premium Bonds, Betting winnings, and Lottery winnings.

You complete the wrong box or boxes

There are a lot of boxes on a UK tax return. Therefore, it is important that you are very careful when you complete this. In most cases, a business owner will let their accountant take care of their tax return.

You forget to include supplementary pages 

Another SA error or mistake is overlooking to include supplementary pages with your return. If you have other income, you may need to submit extra pages with this. These extra pages will include:

  • Interest from gilt-edged and other UK securities.
  • Life insurance gains.
  • Share schemes.
  • Employment lump sums or any compensation payments from a previous employer.
  • Stock dividends, non-qualifying distributions, and loans that you write off.

Forget to sign and date your return 

You may be one of the few that still send your return in through the post by 31 October. This date is three months earlier than the online filing deadline of 31 January. If you do this, please make sure that you sign and date the declaration before you send this to HMRC.


HMRC will charge penalties for late or inaccurate tax returns. They also charge penalties when a taxpayer pays the tax that is due late. Therefore, it is key to make sure that you get this right.

If you find that you are unable to pay your penalties or tax liabilities, you will need to contact HMRC.

Final thoughts 

A Self-Assessment error or mistake can occur if you or your accountant are not careful when you complete your return. Therefore, please ensure sure that you give this a good look over before you or your accountant send it in to HMRC.

Link to Contractor Advice UK group on


Published On: March 14th, 2021 / Categories: Self-Assessment /

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